Free trade agreement (FTA) negotiators increasingly face pressure from domestic interest groups, including environmental non-governmental organizations (NGOs), civil activists and labor unions. As a result of the growing scrutiny on the content of FTAs, we are now witnessing a proliferation of instruments accompanying FTAs, which we group under the label of flanking measures. In this article, we argue that flanking measures can serve two main non-exclusive purposes: Increasing aggregate social welfare by mitigating the negative spillovers of FTAs on society (the substantive dimension) and helping to build domestic coalitions in support of trade liberalization (the political dimension). Despite the relevance and growing empirical importance of the concept, flanking remains largely overlooked in the International Political Economy (IPE) literature. This essay seeks to fill this gap by discussing the scope, purposes and timing of flanking.